Short wave diathermy circuit

ABSTRACT

A diathermy having a vacuum tube in a tuned plate, tuned grid oscillator circuit. A coil is placed in the tuned plate circuit for applying short wave signals to a patient undergoing treatment. The tube has at least a control grid connected to the tuned grid circuit and a screen grid. A circuit is coupled to the screen grid for enabling a signal to be fed back from the plate circuit through the screen grid to the control grid and comprises an impedance for coupling the screen grid to a source of control signals for controlling the power delivered by the coil to a patient undergoing treatment.

United States Patent Mettler 1 Feb. 1, 1972 [541 SHORT WAVE DIATHERMY CIRCUIT 211 Appl. No.: 846,012

2,321,659 6/1943 Connerty et al. 2,695,021 1 1/1954 Touzel 3,478,744 11/1969 Leiter ..128/303.14

Primary Examiner-William E. Kamm AtlorneyChristie, Parker & Hale [57] ABSTRACT A diathermy having a vacuum tube in a tuned plate, tuned grid oscillator circuit. A coil is placed in the tuned plate circuit for applying short wave signals to a patient undergoing treatment, The tube has at least a control grid connected to the tuned grid circuit and a screen grid. A circuit is coupled to the screen grid for enabling a signal to be fed back from the plate circuit through the screen grid to the control grid and comprises an impedance for coupling the screen grid to a source of control signals for controlling the power delivered by the coil to a patient undergoing treatment.

11 Claims, 2 Drawing Figures l Iii i [Ill {56] References Cited UNITED STATES PATENTS 2,186,434 1/1940 Senauke ..128/422 2,276,996 3/1942 Milinorski. .....l28/422 2,311,631 2/1943 Bach ..331/182 l \/V \/V\ /48 5CIPN CONTROL VOL7216E SHORT WAVE DIATHERMY CIRCUIT BACKGROUND OF THE INVENTION 1 Field of the invention This invention relates to diathermy-circuits and, more particuiarly, to an improved control for a tuned plate, tuned grid oscillator for generating short wave diathermy signals.

2. Description of the Prior Art Short wave diathermies are known for applying highfrequency electromagnetic waves through the tissues of a patient for therapeutic purposes. One type has a single tube and pads for applying the diathermy signals to the patient. How-- ever, the harmonics formed by this device are prohibitive and the circuit requires a very high ratio of capacitance to inductance necessitating a very high idling current. An amplifier-oscillator type of circuit is also known having a patient resonating circuit. The amplifier-oscillator type of prior art diathermy is much more costly than the first device. Both of the aforementioned devices are very sensitive and require precise tuning. Mere breathing of the patient detunes the device. Technicians operating these devices have a very difficult time tuning them.

To alleviate the problem of tuning, one prior art diathermy has a tracking circuit to keep the circuit in tune, but it is much more costly. All of the aforementioned devices suffer from the disadvantages that .large harmonics are generated and a faraday shield or other isolation circuit is required to shield the harmonics formed in the oscillator circuit from the patient resonating circuit.

All of the aforementioned diathermies sufier from the deticiency that the power being delivered to the patient cannot be measured and indicated. This problem arises for a number of different reasons, all related to the design of these devices. For example, patient .pads or cable electrodes used to apply the diathermy energy to the patient use long leads which are required to couple the patient electrodes to the oscillator and, as a result, energy is radiated into the atmosphere from the leads. There is no way of measuring the power being radiated and, hence,- the power delivered to the leads cannot be used as a measure of power delivered to the patient.

An improved short wave diathermy circuit has been proposed in which the coil in the tuned plate circuit of the vacuum tube oscillator is used to directly apply short wave diathermy signals to the patient undergoing treatment. In this manner, the patientresonating circuit can be eliminated, thus eliminating the problem of keeping the circuit tuned. However, the improved circuit has never been used commercially because it generates a prohibitive amount of energy at harmonic frequencies related to the fundamental frequency being applied to the patient. These harmonic frequencies interfere with communications and cannot be tolerated as they conflict with the rules laid down by the Federal Communications Commission with respect to .the band of permissible frequencies forthis type of equipment.

SUMMARY OF THE INVENTION The present invention is directed to an improvement over the aforementioned diathermies in which signals are-applied directly from the coil in the tuned plate circuit to the patient. The broad concept of the present invention involves means connected to the screen grid for enabling the fundamental frequency of oscillation in the plate circuit to be fed back from the plate circuit through the screen grid to the corresponding control grid and comprises an impedance coupled to the screen grid. Preferably, a source of screen control voltage is connected through the impedance to the screen grid for controlling the power output of the oscillator circuit. As a result, the harmonics formed by the circuitare lowered to a tolerable level. Using the present invention, it has been found that the output of the power oscillator circuit can be precisely controlled using the source of screen control voltage.

A preferred embodiment of the invention has a pair of tubes in a push-pull tuned plate tuned grid oscillator circuit, each tube having a screen control grid. An impedance couples each control grid to a source of control signals for the circuit.

A preferred form of the invention utilizes a beam tube for .the vacuum tube.

In summary, a short wave diathermy in accordance with the present invention has marked advantages over the aforementioned prior art short wave diathermies. For example, the troublesome harmonics which interfere with communications are virtually eliminated, and the power output can be precisely controlled using the screen control voltage. All body areas of a patient undergoing treatment can be treated by appropriate setting only of the screen control voltage. The device is always in tune and hence no tuning is required. Additionally, there is no power loss from patient or cable radiation. High-voltage plate transformers, expensive power tubes and rectifier tubes are not required. Additionally, the oscillator and patient coil can be housed right in the treatment head of the device which allows the long dangling cables of the patient electrodes to be eliminated.

Another important feature is that a meter circuit can be added in series with the power supply for the circuit and can be used to indicate true power being drawn by the patient undergoing treatment.

Although the invention is primarily adapted for and has highly unusual features and advantages for use in combination in a short wave diathermy, it is felt that the invention in the oscillator circuit has wider applications in other applications, including, but not limited to, induction heaters and radio frequency generators and is within the broader scopes of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of a short wave diathermy circuit and embodying the present invention; and

FIG. 2 is a schematic diagram showing a prior art short wave diathermy unit.

DESCRIPTION OF THE PREFERRED EMBODIMENT Before considering the present invention, consider the prior art diathermy circuit presently used commercially, as shown in FIG. 2. The prior art diathermy circuit employs a conventional tuned plate, tuned grid circuit employing a pair of triodes connected in a conventional push-pull relationship well known in theelectronic circuit art. A tuned plate, tuned grid circuit of the type shown in FIG. 2 is shown in FIG. 112(g) of the book entitled Radio Engineering by Terman published by the McGraw Hill Book Company, Inc. New York and London I932. To the right of the push-pull tuned grid, tuned plate circuil is a patient resonating circuit 12. The patient resonating clrcuit includes a tuned circuit having a coil 16 magnetically coupled to the coil in the plate circuit of the oscillator I 0 and a variable capacitor 1 8. The coil 16 and capacitor I 8 are connected in parallel. EAch side of the parallel connection of the coil and capacitor is connected to patient pads, cable, or drum I 4 a )and 14(b )through inductors 20 and 22. Control over the power applied to the patient is achieved by tuning the patient resonating circuit to just off the oscillator frequency. This adjusts the power coupled into the patient resonating circuit. The coupling between coil 16 and the oscillator plate circuit coil is fixed. A faraday shield is provided in between the oscillator circuit 10 and the patient resonating circuit 12.

Consider now the present invention as shown in FIG. 1. The circuit includes a helical induction coil 30 for applying the short wave diathermy signals directly to the patient undergoing treatment.

The center tap of the coil 30 is connected to 8+ source of potential. The ends of the coil 30 are connected across two series connected capacitors 32 and 34. The coil 30 and capacitors 32 and 34 form a tuned tank circuit for the plate circuit of the oscillator. The junction between the two capacitors 32 and 34 is connected toground potential. It will be evident to those skilled in the art that each capacitor 32 and 34 may comprise two or more capacitors connected in parallel to make up the desired capacitance.

The two sides of the tank circuit 29 are labeled 29(a and 29(b and are connected to the plate electrodes of a pair of beam vacuum tubes 36 and 38, respectively. Each tube has a plate, a screen grid b, a control grid 0, and a cathode d. It should be understood that the invention is not limited to use of a beam tube but any tube having at least the aforelisted electrodes and similar characteristics may be used.

The cathodes of the pentodes 36 and 38 are connected together to a 8- source of potential.

The tuned grid circuit is shown generally at 41 in FIG. 1. The tuned grid circuit includes an inductive coil 39-40 coupled between the control grids of the two tubes. This coil is center-tapped and the center tap is connected to the cathodes through a resistor 42 which is provided to limit the current passing through the grid coil 39-40. The resistor 42 is very high impedance and hence the cathodes are not directly connected to the center tap. The grid to cathode capacitances (not shown) of the tubes 36, 38 are in series across the ends of the grid coil 39-40 to form the tuned grid circuit.

Resistors 44 and 46 connect the screen grids b of the pentodes 36 and 38, respectively, to a source of screen control voltage 48.

The upper and lower halves of the oscillator circuit shown in FIG. 1 each form a conventional tuned plate, tuned grid circuit of the type described in the above-mentioned hook entitled Radio Engineering". The two halves are connected together in a conventional push-pull fashion well known in the oscillator circuit art. Accordingly, a description of the operation of the oscillator circuit will not be given, and a more complete understanding thereof can be had with reference to the above referenced book entitled Radio Engineering.

However, the control for the power delivered to the patient through the coil 30 should be noted. The amount of power is controlled by the voltage output from the source of screen control voltage 48.

In one embodiment of the invention the screen control voltage 48 varies between and 140 volts, causing power to be delivered by the coil 30 to a patient varying between 0 and 100 watts.

The screen resistors 44 and 46 are of considerable importance to the present invention. It should be noted that the screen resistors 44 and 46 are each of such a value that the screen grids retain their ability to control the power to the circuit and yet large enough to allow the fundamental short wave frequency to be fed back from the plate the screen grid to the control grid and thereby sustain oscillation in the circuit. Most known oscillator circuits of this type are only triodes and do not utilize a screen grid for control. By utilizing a tube with a screen grid and connecting the screen resistors as shown, significantly improved results are obtained by eliminating the undesirable harmonics which are prevented diathermy circuits of this type from being useful in the past.

The values of the resistors 44 and 46 do not appear to be critical and it has been found that resistors may be selected by placing a variable resistor in place of the resistors 44 and 46 and varying them until the desired power output control is obtained. However, it has been found that the undesirable harmonies are filtered out better as the value of resistors 44 and 46 is increased, up to the point where the screen grids lose control of the power output.

Table I shows the values of the various elements shown in the diathermy circuit of FIG. 1 for a preferred embodiment.

Although one example of the present invention has been shown by way of illustration, it should be understood that there are many other rearrangements and embodiments of the present invention within the scope of the following claims.

TABLE I tuned to 27.2 MHz.

tuned to 32.5 MHz.

0.4 microhenries 140 picolarads 34 picofuruds 36 type-6 HFS beam tube 38 type-6 HFS beuni lube 39-40 2.5 micruhenries 48 0 to [40 volts 13+ +600 volts B- 0 volts lclaim:

1. A diathermy comprising a pair of vacuum tubes in a pushpull tuned plate, tuned grid oscillator circuit having a coil in the tuned plate circuit for applying short wave signals to a patient undergoing treatment, the tuned plate circuit oscillating at a fundamental short wave frequency, each of said tubes having at least a control grid connected to the tuned grid circuit and a screen grid, a source of screen control potential and means coupled to each screen grid for enabling said fundamental frequency to be fed back from the said plate circuit through each screen grid to the corresponding control grid and comprising an impedance for each screen grid for coupling the corresponding screen grid to said source of screen control potential for controlling the power delivered by the coil to a patient undergoing treatment.

2. A diathermy according to claim 1 wherein the said tuned plate circuit is tuned to a different frequency than that of said tuned grid circuit.

3. A diathermy according to claim 2 wherein the frequency at which said tuned plate circuit is tuned is lower than that of said grid circuit.

4. A diathermy as defined in claim 1 wherein each of said impedances comprises a resistor.

S. A diathermy as defined in claim 1 wherein said tubes comprise a beam tube.

6. A diathermy comprising a pair of vacuum tubes in a pushpull tuned plate, tuned grid oscillator circuit having a coil in the tuned plate circuit for applying short wave signals to a patient undergoing treatment, the tuned plate circuit oscillating at a fundamental short wave frequency, each of said tubes having at least a control grid connected to the tuned grid cir cuit and a screen grid, a source of screen control potential and means coupled to each screen grid for enabling said fundamental frequency to be fed back from said plate circuit through each screen grid to the corresponding control grid and comprising impedance means coupled between said screen grids and said source of control potential for controlling the potential to the screen grids and thereby control the power delivered by the coil to a patient undergoing treatment.

7. A diathermy as defined in claim 6 wherein said impedance means comprises at least two resistors connected in series between said screen grids and the junction between said resistors being connected to such source of control potential the resistors thereby causing any harmonics to be suppressed.

8. A diathermy as defined in claim 6 wherein said coil is a helical induction coil.

9. A diathermy comprising a push-pull tuned plate, tuned grid oscillator circuit having first and second vacuum tubes, each having at least a control grid and a screen grid, a cathode electrode and a plate electrode, a source of negative potential, the cathode electrodes being coupled to the source of negative potential and the plate electrodes being coupled across a tuned plate tank circuit comprising a helical induction coil which coil is for applying short wave signals to a patient undergoing treatment, and at least two capacitors connected together in series and coupled in parallel with said coil, 21 source of positive potential for the circuit connected to said coil, a reference potential coupled to the junction between said at least two series connected capacitors, a tuned grid circuit comprising a tank circuit having a coil coupled between the two control grids and impedance means for coupling the cathodes of said tubes to the coil in the tuned grid circuit, a resister for each screen grid, each resistor having one end coupled to the corresponding screen grid and a source of screen control potential coupled to the other end of said resistors for controlling the power applied to the patient 10. A diathermy comprising at least one vacuum tube in a tuned plate, tuned grid oscillator circuit having a coil in the tuned plate circuit for applying short wave signals to a patient undergoing treatment, the tuned plate circuit oscillating at a fundamental short wave frequency, said tube having at least a control grid connected to the tuned grid circuit and a screen grid, a source of screen control potential and means coupled UNITED STATES PATIENT OFFICE Patent No.

February 1, 1972' Inventor'(s) Dated Hal C. Mettler It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 2, line Col. 3, line line line line- Col. 4,line

57, "'la)" shouldread 6 "a" should be inserted after "plate";

17, "a" should be inserted after --is--; 48, after late" insert --circ uit through--; 54, delete 'are",

32, after "said" insert -tu ned--Q Signed and sealed this 1 1 th day of July 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Attesting Officer I ROBERT GOTTSCHALK Commissionerof Patents 

1. A diathermy comprising a pair of vacuum tubes in a push-pull tuned plate, tuned grid oscillator circuit having a coil in the tuned plate circuit for applying short wave signals to a patient undergoing treatment, the tuned plate circuit oscillating at a fundamental short wave frequency, each of said tubes having at least a control grid connected to the tuned grid circuit and a screen grid, a source of screen control potential and means coupled to each screen grid for enabling said fundamental frequency to be fed back from the said plate circuit through each screen grid to the corresponding control grid and comprising an impedance for each screen grid for coupling the corresponding screen grid to said source of screen control potential for controlling the power delivered by the coil to a patient undergoing treatment.
 2. A diathermy according to claim 1 wherein the said tuned plate circuit is tuned to a different frequency than that of said tuned grid circuit.
 3. A diathermy according to claim 2 wherein the frequency at which said tuned plate circuit is tuned is lower than that of said grid circuit.
 4. A diathermy as defined in claim 1 wherein each of said impedances comprises a resistor.
 5. A diathermy as defined in claim 1 wherein said tubes comprise a beam tube.
 6. A diathermy comprising a pair of vacuum tubes in a push-pull tuned plate, tuned grid oscillator circuit having a coil in the tuned plate circuit for applying short wave signals to a patient undergoing treatment, the tuned plate circuit oscillating at a fundamental short wave frequency, each of said tubes having at least a control grid connected to the tuned grid circuit and a screen grid, a source of screen control potential and means coupled to each screen grid for enabling said fundamental frequency to be fed back from said plate circuit through each screen grid to the corresponding control grid and comprising impedance means coupled between said screen grids and said source of control potential for controlling the potential to the screen grids and thereby control tHe power delivered by the coil to a patient undergoing treatment.
 7. A diathermy as defined in claim 6 wherein said impedance means comprises at least two resistors connected in series between said screen grids and the junction between said resistors being connected to such source of control potential the resistors thereby causing any harmonics to be suppressed.
 8. A diathermy as defined in claim 6 wherein said coil is a helical induction coil.
 9. A diathermy comprising a push-pull tuned plate, tuned grid oscillator circuit having first and second vacuum tubes, each having at least a control grid and a screen grid, a cathode electrode and a plate electrode, a source of negative potential, the cathode electrodes being coupled to the source of negative potential and the plate electrodes being coupled across a tuned plate tank circuit comprising a helical induction coil which coil is for applying short wave signals to a patient undergoing treatment, and at least two capacitors connected together in series and coupled in parallel with said coil, a source of positive potential for the circuit connected to said coil, a reference potential coupled to the junction between said at least two series connected capacitors, a tuned grid circuit comprising a tank circuit having a coil coupled between the two control grids and impedance means for coupling the cathodes of said tubes to the coil in the tuned grid circuit, a resistor for each screen grid, each resistor having one end coupled to the corresponding screen grid and a source of screen control potential coupled to the other end of said resistors for controlling the power applied to the patient.
 10. A diathermy comprising at least one vacuum tube in a tuned plate, tuned grid oscillator circuit having a coil in the tuned plate circuit for applying short wave signals to a patient undergoing treatment, the tuned plate circuit oscillating at a fundamental short wave frequency, said tube having at least a control grid connected to the tuned grid circuit and a screen grid, a source of screen control potential and means coupled to said screen grid for enabling said fundamental frequency to be fed back from said plate circuit through said screen grid to said control grid and comprising an impedance for coupling the screen grid to said source of control potential for controlling the power delivered by the coil to a patient undergoing treatment.
 11. A diathermy according to claim 10 wherein said tube comprises a beam tube. 